In a disc drive using magnetic recording media, data is recorded in concentric tracks on a plurality of disc surfaces. The recording heads are mechanically ganged together and are arranged one per surface with all heads nominally positioned over the same radial track location. The assembly of heads are positioned in unison using a voice coil motor. The recorded data, consisting of a certain topology of magnetic transitions are recorded on the surface, or surfaces, of the magnetic media. The reading and writing of the recorded data is accomplished by read and write heads that are positioned on the required track by the drive's positioning control system. During operation of the disc drive, the heads fly over the media. The fly height depends on the drive design and can be very small, typically sub micro-inch to micro-inches. In addition, the texture of the media can be rough depending upon the media used. As a result of the physical properties of the elements involved and of the operating conditions, head to disc interference, or contact may occur. The interference is significant because of the impact on the principles involved in reading the data. The magnetic transitions emanate magnetic fields from the magnetized regions on the disc (disk). When a magnetized region passes under a head, flux reversals are detected to produce an analog signal (a voltage pulse) which is converted to a digital signal by the drive's read electronic circuitry. The shape of the voltage pulse and its ability to be recovered depends on the type of media coating and the fly height, sometimes also referred to as the distance from the read head gap. The fly height is a function of the head-surface separation and the depth of the flux reversals within the coating of the magnetic media. When head-disc interference occurs, air bearing vibrations in the 120 to 250 Khz range are present. The vibration in the air bearings leads to slider motion in the direction along the data track. This motion causes the transducer to move back and forth along the track during the read back operation. Such back and forth movement, or oscillation, of the head in the longitudinal axis causes the head to read magnetic flux transitions earlier and later, which cause frequency modulation in the read back signal. The modulation is proportional to the amount of slider vibration, which is a reflection of the severity of the head-disc interference. The more severe the interference, the more severe the modulation. The interference may be slight or severe, depending upon the amount of clearance that can be established by the air bearing supporting the slider. Wear or burnishing of the transducer and the alumina at the back of the slider typically will occur, see generally FIG. 2. The continued interference leads to generation of debris, contamination of the slider and consequently, degradation of the read signal from the drive. If such interference persists and the condition is not checked, this eventually leads to disastrous head crashes and permanent data loss.
The known methods and apparatus used for analysis of head-disc interference during drive operations include the use of acoustic emission (AE) sensors and laser doppler vibrometers to detect mechanical vibrations. In using the AE sensor and the laser doppler vibrometer, the target components are inside the head gimbal assembly (HGA). In the case of the AE sensor, the device must be mounted on the E-block member of the HGA. Any mechanical vibration on the HGA during drive operation induces (or transduces) a corresponding electrical signal at the output of the AE sensor. In using the laser doppler vibrometer, the laser is pointed at the slider within the head disc assembly (HDA) containing the HGA to detect vibrations on this component. The use of the AE sensor and the laser doppler vibrometer involve an invasive procedure that requires opening the HDA which has inherent disadvantages of introducing contamination into the assembly. The contaminations are minimized by conducting the procedure in a clean room environment. The mounting of the AE sensor is typically accomplished using glue which introduces contamination, not only at time of mounting, but also at time of removal of the sensor element. If a number of head disc assemblies are suspected of having head-disc interference, each must be invasively analyzed. The signal produced at the output of the AE sensor, or the vibration detected by the laser doppler vibrometer, can be caused by a number of factors which, although minimal, the potential for erroneous oscillation sources exists. For example, inadvertent vibration of the test bench, HDA or other elements not associated with the HGA could be erroneously identified as an oscillation source. Further, in using these methods, the actual head that is experiencing longitudinal oscillations can not be particularly identified. Mechanical vibration detection methods are viewed as being limited in that all modes of vibration, i.e. longitudinal, lateral and vertical vibrations contribute to the response of the AE sensor, or laser doppler vibrometer, and as such, the amount of oscillation due only to the longitudinal axis cannot be determined.
To applicants' knowledge, there are no known apparatus and method which capitalize on the existence of frequency modulation that is created by head-disc interference during reading of recorded magnetic transitions to analyze the severity of the head-disc interference.
Therefore, there is a need for an improved method and apparatus for determining the existence, severity and mode of head and media interference so as to take corrective measures against generation of debris, contamination of the slider and consequently, degradation of the read signal from the drive and to prevent disastrous head crashes and permanent data loss. In particular, there is a need for apparatus and method which accurately and precisely detects and measures longitudinal axis vibration of a head that is caused by head-disc interference.
It is therefore a primary object of the present invention to provide a method and apparatus for detecting head-disc interference for purposes of determining the existence of head-disc interference so as to take preventative and corrective measures to prevent disastrous head crashes and permanent data loss by capitalizing on the frequency modulation that is generated by head-disc interference.
It is another related object of the present invention to provide a disc drive apparatus and method for detecting and measuring longitudinal axis (along the track) head-disc interference for purposes of determining the severity of the head-disc interference so as to take preventative and corrective measures to prevent disastrous head crashes and permanent data loss by capitalizing on the frequency modulation that is generated by head-disc interference.